The present invention relates generally to systems and methods for controlling an electric motor and, more particularly, to a controller and associated system and method for switching noise reduction of a pulse-width-modulated controlled brushless direct current (BLDC) motor.
Electrical machines are used throughout a great number of devices today, and typically consist of motors, which convert electrical energy into mechanical energy, and generators, which convert mechanical energy into electrical energy. Generally, electrical machines fall into one of three categories: polyphase synchronous machines, polyphase asynchronous (i.e., induction) machines and direct current (DC) machines. Typical machines consist of two main portions: a stationary, outside portion called a stator, and a rotating, inner portion called a rotor. The rotor of typical machines is mounted on a stiff rod, or shaft, that is supported in bearings so that the rotor is free to turn within the stator to produce mechanical energy.
In one type of synchronous machine, a permanent magnet, brushless direct current (BLDC) machine, the stator is composed of windings that are connected to a controller, and the rotor is composed of two or more permanent magnets of opposed magnetic polarity. The controller generates poly-phase alternating input currents to the stator windings. As the rotor rotates within the stator, and the magnets of one polarity approach cores that conduct the opposed polarity, sensors signal the angular position of the rotor to the controller which, in turn, controls the alternating currents to switch the polarity of the magnetic field produced by windings on the stator. For example, a three-phase BLDC motor can have two, four or more permanent magnets with alternating magnetic polarities mounted on its rotor. The required rotating magnetic field is produced by current through the stator windings. And the three phases of the current are switched in sequence, which is dictated by the angular position of the rotor.
In many BLDC motor systems, the speed of the BLDC motor is controlled by pulse modulating, such as pulse width modulating, the input voltage generated by the controller. By pulse-width-modulation (PWM) of the input voltage, the controller controls the average input currents to the windings by using xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d states. During the time the input currents through the windings are increasing, the voltage supply provides constant voltage to the controller at a level at least the as high as the motor voltage required for the desired speed of operation. Once the currents have reached the required levels for the desired speed of the motor, the duty cycle is changed to that required to maintain the currents at or near the required level of current.
While PWM can provide an acceptable method of controlling the speed of a BLDC motor, it has some drawbacks. Among the drawbacks, in addition to producing currents at the desired rotational frequency, modulating the voltages produces in the system an unwanted current ripple at the PWM switching frequency and at higher harmonics of that frequency. The ripple current is a superimposed current on the average input currents to the windings, generated as the system keeps the input currents centered about an average value. The ripple current disadvantageously manifests itself as electromagnetic interference (EMI) and causes vibration noise in the system in the form of mechanical motion (i.e., rotational and megneto-striction) within the motor.
The amount of ripple current, Iripple, produced in the system generally depends upon the switching, fundamental frequency, xcfx89, the voltage across the motor inductance, VL, and the amount of motor inductance, L, as shown in equation (1):                               I          ripple                =                              V            L                                L            xc3x97            ω                                              (        1        )            
As illustrated by equation (1), the ripple current can be reduced by increasing the fundamental frequency or the motor inductance. But these options are costly and have a large schedule impact to the system beyond the motor itself. Also, these options cannot generally be implemented in many current motor controllers as these current motor controllers are not typically manufactured in high drive frequency configurations. Another possible option to reducing the ripple current is to filter the EMI and, therefore, the noise out of the system at the frequencies where the controller produces the ripple current, which effectively increases the motor inductance. But because many motor systems are subjected to large currents and voltages, implementing filters in the system would require costly, robust filters with high current and voltage tolerances.
In light of the foregoing, the present invention provides an improved controller and associated system and method for controlling a brushless direct current (BLDC) motor. The system of the present invention operates with a pulse-width-modulation (PWM) controller to reduce ripple current by controlling the voltage provided to the controller from a voltage source. By controlling the voltage provided by the voltage source, the controller can limit the amount of voltage produced at the motor inductance, which limits the ripple current, which limits the EMI and, thus, the noise produced in the system. Advantageously, the system reduces the ripple current without adding large and costly filters, forcing expensive changes to the controller design, or impacting the schedule for delivery of these systems.
According to one embodiment, the system for controlling the BLDC motor includes a power supply having a controllably alterable voltage output, and a controller in electrical communication with the power supply and the motor. The controller receives the voltage output of the power supply and can provide a pulse-width-modulated input voltage to the motor. Additionally, the controller can measure an average input current to the motor and a speed of the motor and, thereafter, alter the voltage output of the power supply based upon the average input current to the motor and the speed of the motor. In a further embodiment, the system can include an acoustic coating disposed about an outer surface of the motor and the controller.
In another embodiment, the controller includes a drive element and a processing element, with each in electrical communication with the power supply and the BLDC motor. In this embodiment, the drive element receives the input voltage from the power supply. The drive element is capable of providing the pulse-width-modulated input voltage to the motor and measuring the average input current to the motor and the speed of the motor. The processing element is capable altering the voltage output from the power supply based upon the average input current to the motor and the speed of the motor as measured by the drive element. Also, the controller can include a power-factor corrected converter, electrically connected between the power supply and a prime electrical power source that provides power to the power supply. By including the power-factor corrected converter, the efficiency of the power drawn from the prime power source is maximized, while conducted emissions from the system is minimized.
In operation, a controllably alterable voltage is supplied from the power supply to the controller. The controller, in turn, supplies a pulse-width-modulated input voltage to the BLDC motor. As the input voltage is supplied to the motor, the controller measures an average input current to the motor and a speed of the motor. Based upon the average input current to the motor and the speed of the motor, the controller then alters the input voltage from the power supply. For example, the controller can alter the input voltage so that a voltage applied to the motor equals an overhead voltage plus an offset voltage. The overhead voltage depends upon at least one characteristic of the motor, such as a predetermined speed of the motor and/or a predetermined start-up torque of the motor; and the offset voltage depends upon the rates of change of the average input current to the motor and the speed of the motor. In one embodiment, the overhead voltage plus the offset voltage is not more than 40 volts above a terminal voltage across the motor.